Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Restricted cash	$ 27,662	$ 45,018	$ 59,767	$ 62,651
Lender Reserves [Member]
Cash and Cash Equivalents [Line Items]
Restricted cash	26,400	40,700
Restricted Lockbox Accounts [Member]
Cash and Cash Equivalents [Line Items]
Restricted cash	$ 1,300	$ 4,300